Increasing requirements for hydrolysis resistance of polyesters, especially those requirements proposed by the automotive industry, have made it increasingly important to be able to provide thermoplastic polyester compositions having even better resistance to hydrolysis compared to currently available compositions or those described in the existing art.
Hydrolysis resistance of thermoplastic polyesters can be improved by the addition of an epoxy material. However, when incorporated at the high levels necessitated by current hydrolysis resistant requirements, an epoxy material often has the disadvantage of increasing melt viscosity or even increasing the rate of viscosity increase during melt processing, both of which are detrimental to performance in melt fabrication operations such as injection molding.
Japanese Patent Application No. 09208816 A discloses a composition containing, inter alia, polyester resin (particularly of the ethylene terephthalate type), a compound containing at least two epoxy groups and/or an epoxy resin, and carbon black. However, this reference does not specifically disclose in any of the examples the use of both an epoxy resin and epoxy compound, does not disclose the relative ratios of epoxy resin to epoxy compound, if both are to be used, and does not disclose improved hydrolysis resistance when using both.
U.S. Pat. No. 5,596,049 discloses a composition containing, inter alia, linear polyester and difunctional epoxy compounds, particularly those having at least one of the epoxides on a cyclohexane ring. A potential drawback to using cyclohexane ring-based epoxides, however, is the high volatility of such epoxides that are currently available.
It is desirable to obtain a polyester composition that has improved hydrolysis resistant properties while avoiding the above-described drawbacks.
Several patent documents disclose the use of various borates in polyester compositions:
U.S. Pat. No. 4,713,407 discloses a composition comprising, inter alia, 100 wt % polyethylene terephthalate, 10–25 wt % halogenated flame retardant, and 3–10 wt % zinc borate. It is disclosed that the polyester composition is used for mouldings for car parts and home electric appliances and has flame resisstance combined with heat resistance, moldability, and good mechanical properties.
JP 09194699 A (Abstract) recites a thermoplastic resin composition comprising a composition comprising, inter alia, polybutylene terephthalate; a composition comprising, inter alia, glass fibres; and a composition comprising, inter alia, zinc borate. Disclosed advantages are improved mechanical properties and little warpage, giving molded articles having high dimensional accuracy.
WO9634047 A1 discloses a flame-retardant composition comprising polyester; an oxalate compound; and an inorganic compound selected from a group including borates.
JP 08142510 A (Abstract) recites a housing material for laser marking comprising a thermoplastic resin composition which is prepared by mixing 0.001–10 wt % of borate with a thermoplastic resin. The recited advantage is that a sharp black marking can be performed without inhibiting the balance among the tracking resistance, the flame retardant, and the dielectric breakdown strength.
JP 06025463 A (Abstract) discloses an anti-inside blocking additive comprising aluminum borate, preferably in particulate form having an average particle diameter of 0.1–50 microns or in lamellar crystalline form. A resin film, such as film made from polyethylene terephthalate, may contain this additive. The resulting advantages, it is disclosed, are excellent sliding and anti-inside blocking properties.
However, none of the above-reference patent documents disclosing borates is directed to improving hydrolysis resistance.
JP 07102153 A (Abstract) recites a composition containing a thermoplastic polyester resin, an epoxy resin, a silane coupling agent and an inorganic filler. In an example, it is disclosed that a mixture of polybutylene terephthalate and novolak epoxy resin were mixed with aluminum borate whisker, which had a fiber diameter of 0.5–1.0 microns and a length of 10–20 microns. JP 62041967 B (Abstract) recites a flame-retarding resin composition comprising polyester; a very specific modified epoxy compound; inorganic flame-retarding auxiliary agent; and reinforcing agent. The inorganic flame-retarding auxiliary agent may be barium metaborate. Neither of these abstracts, however, discloses the use of those compositions for hydrolysis resistant applications.